Corrosion-resistant multilayer coatings

ABSTRACT

A corrosion-resistant coating for a substrate is described. The corrosion-resistant coating comprises a first distinct layer of a first composition disposed over the substrate, wherein the first distinct layer has a thickness that is not greater than about 10 microns, and a second distinct layer of a second composition disposed over the first distinct layer, wherein the second distinct layer has a thickness that is not greater than about 10 microns and either the first distinct layer or the second distinct layer is corrosion-resistant. Preferably, the thickness of each distinct layer is less than about 1 or 2 microns, more preferably, less than about 0.4 microns. The coating may comprise additional layers. Corrosion-resistant articles, methods of protecting an articles, and methods of depositing corrosion-resistant coatings are also described.

TECHNICAL FIELD

This invention relates to corrosion-resistant coating compositions and aprocess for applying multilayer coatings. More particularly, thisinvention relates to thin, multiple layer coatings composed of layers ofat least two differing compositions. Preferably, the multiple layers ofthe coating are applied by a combustion chemical vapor depositionprocess. The coatings can be applied to the surfaces of various articlesin order to provide beneficial surface properties to the articles.

BACKGROUND OF THE INVENTION

The use of coatings to provide corrosion protection to an underlyingarticle or substrate is common. Protective coatings can include organiccoatings such as paints and epoxies; nonmetallic coatings such ascements, enamels and oxides; and metallic coatings such as chrome andgold plating. Application of these coatings can be accomplished by suchdiffering processes as painting and spraying to plating and vapordeposition. The process of applying the coating is often dependent on orlimited by the properties of the material being deposited and of theproperties of the substrate.

Research on improving protective coatings has been extensive for manydifferent materials and applications. The object of protective coatingsis to provide corrosion resistance to the underlying substrate andenhance the corrosion resistance of the substrate to the variousenvironments the substrate may encounter. Many coatings are limited toparticular environments because of their inability to withstand certaintemperature and/or corrosive conditions. The use of organic binders inmany coatings limits the use of those coatings at elevated temperatures.A coating not requiring organic binders may withstand elevatedtemperatures.

Additionally, many articles requiring corrosion protection have specificweight limitations. Therefore, thinner and accordingly lighter coatingsare desired. Thinner coatings are also desirable because they requireless material, do not significantly change the substrate size, and offerthe potential to reduce material costs. Protective coatings, regardlessof their composition and the manner in which they are applied, must beadherent to the substrate they are to protect. In order to protect theunderlying substrate, the protective coatings must act as a protectivebarrier against the corrosive agent or as a sacrificial layer.Sacrificial protective layers have the disadvantage that a sacrificialprotective layer only provides temporary protection and must be replacedonce it has been expended.

Inorganic coatings have also been used for corrosion protection.However, inorganic coatings are typically made of materials that havelow coefficients of the thermal expansion relative to the highercoefficient of thermal expansion metal substrates they are intended toprotect. While inorganic coatings may perform adequately at a particulartemperature, the inorganic coatings on the metal substrates are not ableto withstand large temperature changes. When the metal substrate and thecoating are subject to large temperature increases and decreases, theunderlying metal substrate expands and contracts, respectively, to agreater degree than the inorganic coating. The coefficient of expansionmismatch causes the brittle inorganic coating to crack and break awayfrom the surface of the metal, a phenomenon known as spalling. Thus, themetal is no longer protected by the coating and may become exposed tothe corrosive agents.

Metals have been used as protective coatings. However, most metals aresubject to corrosion, especially at elevated temperatures and inaqueous, salt and acidic environments. Additionally, metal coatings areexpensive, heavy and can be removed by abrasion.

Accordingly, there is a need for an improved corrosion-resistant coatingwhich is more durable and effective under a broader range of conditions,particularly at elevated temperatures and in acidic and salineenvironments.

SUMMARY OF THE INVENTION

The present invention fulfills the above described needs by providing amultilayer coating for an article or substrate and a method ofprotecting article substrates. The coating and coated article comprise acoating system which comprises a first distinct layer of a firstcomposition over a metal substrate and a second distinct layer of asecond composition over the first distinct layer, wherein thecomposition of the second distinct layer is different than thecomposition of the first distinct layer. The coating system may compriseadditional layers.

A corresponding method of protecting a substrate or article is alsodisclosed. The method comprises the steps of depositing a first distinctlayer of a first composition over the substrate and depositing a seconddistinct layer of a second composition over the first distinct layerwherein the composition of the second distinct layer is different thanthe composition of the first distinct layer. Preferably, the multiplelayers of the coating are deposited by a combustion chemical vapordeposition process. In a first preferred embodiment, the coatingcomprises alternating, discrete layers of silica and chromia. In asecond preferred embodiment, the coating comprises alternating, discretelayers of silica and zinc phosphate, wherein the silica layer may be adoped or undoped layer of silica. In a third preferred embodiment, thecoating comprises alternating, discrete layers of silica and zincphosphate, wherein the silica layer may be a doped or undoped layer ofceria.

Accordingly, an object of the present invention is to provide animproved corrosion-resistant coating.

Another object of this invention is to provide an improved method ofprotecting a corrosion susceptible substrate or article.

Still another object is to provide a coating system that is economicaland easy to apply.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description, drawings,examples and claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Metals, in particular non-noble metals, are susceptible to corrosion.Corrosion-resistant coatings have been applied to the surface of metalsto protect the metal from the corrosive agent(s). The coatings must beable to withstand the corrosive agents and any environmental conditionswhich the coating and underlying metal are likely encounter. Two typesof corrosion-resistant coatings currently used include organic coatings,such as plastic coatings and paints, and inorganic coatings. The organiccoatings, while typically easy to apply, do not always providesufficient protection in all environmental conditions. Organic coatingsand coatings containing organic components degrade or melt at hightemperatures and therefore are not able to withstand elevatedtemperatures. The multilayer coatings in accordance with the presentinvention do not require organic components which degrade or melt athigh temperatures. Additionally, the multilayer coatings in accordancewith the present invention do not require the use of any intermediatecompounds such as binders, etc. to add additional weight and cost orlimit the use of the coatings at high temperatures. The multilayercoatings in accordance with the invention are better able to withstandelevated temperatures and liquid corrosive environments thanconventional coatings.

Although, inorganic coatings are better able to withstand elevatedtemperatures they are more difficult to apply than organic coatings.Additionally, inorganic coatings, such as aluminum oxide, silicondioxide, chromium oxide, etc., typically have low coefficients ofthermal expansion relative to the metals, steel, aluminum, copper,brass, etc., upon which they are coated to protect. When the metalsubstrates and the overlying inorganic coatings are subject to largetemperature changes, the metal substrate expands to a greater degreethan the overlying inorganic coating. The coefficient of expansionmismatch causes the brittle inorganic coatings to crack and break awayfrom the surfaces of the metal substrate, a phenomenon referred to asspalling. Thus, the metal substrate is no longer protected by theinorganic coating and may become exposed to the corrosive agents.

The present invention teaches an improved coating system that is betterable to withstand elevated temperatures and with improved corrosionresistance. The improved performance of the coatings is obtained byusing thin layers which are better able to withstand the largetemperature changes and by using multiple layers which provide improvedcorrosion resistance. Such coating systems are useful for protectingmost metal substrates, including aluminum, iron, copper, nickel, andtitanium alloys. Also, composites containing metals can be protected ora metal substrate of any material with a metal coating thereon can besimilarly protected. As used herein the term “substrate” is intended toinclude articles. The coatings are particularly useful for articleswhich are used at elevated temperatures, such as turbines that aresubject to large temperature changes and corrosive combustion gases butcan still be beneficial for substrates which are never exposed toelevated temperatures.

The multilayer coatings can be used as a primer under additionalcoatings, as a finish coating or include within its structure anon-nanolaminate layer, for example a non-nanolaminate layer of a metalsuch as gold or nickel to provide a desirable, aesthetic finish. Theoptional, non-nanolaminate can vary in thickness and position and can bebelow, between or over the nanolaminate layers of a coating of theinvention. A base adhesion treatment of the substrate such as silanetreatment, cleaning, polishing, etching and other various treatments aredesirable to ensure better bonding and performance of the nultilayercoating. In particular, the substrate should be cleaned of debris andoils to improve bonding.

The coating systems in accordance with the present invention comprise atleast two thin, distinct layers of two differing compositions. In analternative embodiment, the second layer comprises a composition with acoefficient of thermal expansion that is intermediate to the lowercoefficient of thermal expansion of the composition of the first layerand the higher coefficient of thermal expansion of the substrate. Thecoating systems may include any number of additional layers, with fewerlayers being more economical and easier to apply. Coating systemsconsisting of four, five and six distinct layers of alternatingcompositions of silica and chromia are demonstrated in the followingexamples Coating systems consisting of two and five distinct layers ofalternating compositions of doped or undoped silica and zinc phosphateand systems consisting of four distinct layers of alternatingcompositions silica and ceria are also demonstrated in the examples.

Coating systems comprising thinner layers are preferred because thinlayers reduce cost and weight and are preferred in applications in whichweight and cost are critical factors such as components of automobilesand airplanes. Ideally, the layers making up the coating should be asthin as possible while still providing sufficient corrosion protection.A coating system in which each of the individual layers is less than 400nanometers (nm) in thickness is preferred. The term “nanolaminate” asused herein is defined as a laminate or multilayer material of more thanone layer wherein the thicknesses of the individual layers, notincluding the substrate, are measured in nanometers. The termnanolaminate is meant to include laminates of two or more layers whereinthe thickness of the each individual layer not including the substrateis less than 400 nanometers.

One aspect of the present invention involves the composition of thelayers. At least one of the layers should comprise a corrosion-resistantcomposition. Preferably, each layer is useful in corrosion resistance orpassivation. The multilayer coatings of the invention, as demonstratedin the Examples below, have good adhesion to the substrate and excellentcorrosion protection in salt spray tests, in salt water immersion testsand at elevated temperatures. The coatings may be composed of readilyavailable, inexpensive materials that require no special treatmentduring or after their application. The multilayers may be applied tovarious metal substrates which are subject to corrosion, including butnot limited to: aluminum, iron, titanium, tin, copper, nickel and alloysof the previously mentioned metals.

The layers of the coating should comprise at least one layer, preferablytwo layers, of a material whose potential for corrosion is less thanthat of the substrate upon which the layer(s) is to be coated on toprotect. Preferred corrosion-resistant materials do not degrade at hightemperatures and are able to withstand the corrosive conditions andagents which they are likely to encounter such as salt, acids,combustion gasses, humidity, oxygen, etc. Preferred corrosion-resistantmaterials include inorganic oxides. Preferred corrosion-resistantmaterials demonstrated in the Examples include silica, chromia, zincphosphate, ceria and lithia doped silica. The term “silica” as usedherein is meant to include all binary compounds of silicon and oxygen,Si_(x)O_(y), of which the most preferable compound is silicon dioxide,SiO₂. The term “chromia” as used herein is meant to include any binarycompound of chromium and oxygen, Cr_(x)O_(y), of which the mostprevalent is chromium(III)oxide, Cr₂O₃. The term “lithia” as used hereinis meant to include all binary compounds of lithium and oxygen,Li_(x)O_(y), particularly lithium oxide, Li₂O₂. The term “ceria” as usedherein is meant to include all binary compounds of cerium and oxygen,Ce_(x)O_(y), particularly cerium dioxide, CeO₂. Zinc phosphate isZn₃(PO₄)₂. Other known corrosion-resistant materials, such as nitrides,carbides, carbonates, phosphides, phosphates, borides, metals and otheroxides can be similarly effective in a multilayer coating, especially ifnanolaminated.

The formation of multiple layers of thin coatings is beneficial forcorrosion resistance. The total thickness of the combined layers ofcoating should be less than about 40 microns wherein each of theindividual layers is less than about 10 microns thick. Total thicknessesof less than about 10 microns and individual layer thicknesses of lessthan about 2 microns are preferred economically. Total coatingthicknesses of less than 2 microns are preferred and 1 micron areespecially preferred because thermal expansion mismatch between the thincoating layers and the substrate is less detrimental since thinnercoatings and layers are better able to withstand the stresses andstrains generated during temperature changes. When the combinedthickness of the layers of the coating system is greater than onemicron, thermal expansion mismatch is a factor and should be considered.

Multilayer coatings can take advantage of the properties of differentcompositions of the different layers of the coating and combine theminto essentially one useful coating system. Thus, the composition of alayer may be selected to alleviate a problem inherent in one of theother layers of coating. In addition, the presence of multiple layersallows an upper layer of coating to be abraded away without losingsubstrate protection altogether. One embodiment described herein usesthe beneficial properties of both silica and chromia to provideprotective coatings. This silica/chromia system also exhibits theadvantage of allowing deposition without cracks forming during theirdevelopment at elevated temperatures. Another embodiment describedherein uses the beneficial properties of both silica and zinc phosphateto provide protective coatings, especially for brass.

Although not wishing to be bond in theory, it is believed that thepresent invention operates by the following theory. It is known thatwhen materials are formed into very thin films, the materials of thethin films are observed to have different physical properties than thephysical properties of the same materials in bulk. By producing amultilayer material there ire multiple layers of these specializedproperties which can interact with each other and the surroundingenvironment differentially. Further, specialized properties can be madeby combining specific, layered materials and can be optimized forspecific substrates and forms of corrosion. For wet corrosion, thematerials can be optimized for passivation. This can be accomplished byforming an inert and/or absorbing effect surface, in regard to theactive species within the wet environment, to make less susceptible orpassivate the corrosive mechanism, such as active ion species. Since thefilms are so thin the materials should be passivating and not beregarded as cathodic protection, i.e. sacrificial. Another importantconsideration is that the materials of the specific layer can bedeposited as dense inherent coatings at substrate temperatures whichwill not cause substrate deterioration. A preferred method of depositingthe layers of the coating without causing substrate deterioration is acombustion chemical vapor deposition method. Multiple layers alsoprovide protection when one of the layers fails due to an excessivecorrosion event or abrasion.

For elevated temperature protection the minimization of diffusionspecies will provide the greatest protection. Diffusion of corrodingspecies is greatest along grain boundaries. Compound materials tend tohave lower diffusivities than a single material. By nanolaminating, thegrain boundaries are oriented transverse to the undesired diffusionaldirection and thus are not conduits for oxidation or other corrosionspecies. Further, the mode or mechanism of diffusion is changed as itgoes from one material to the next. Thus, nanolaminates can greatlyreduce the net diffusional flux of the corrosion species. Another factorto consider is the effect of corrosion or alteration speciesaccumulating at grain boundaries. Once a high concentration of thecorrosion species is present at a grain boundary or in the present casethe boundary between two materials of the nanolaminate, then thediffusion of the species will be minimized by the presence of such ahigh concentration zone. In fact, since the total coating thickness isprotective at less than 1 micron thickness, diffusion may be a factor ineven wet corrosion at ambient conditions. This theory may explain whymultilayer coatings work better than single layer materials and theexpected cumulative effect of such single layers.

It is also desirable to have one or more of the layers within thenanolaminate to be fracture tough or malleable. The inclusion of afracture tough or malleable layer provides additional wear and abrasionresistance to the substrate. An example of a wear resistant coatingincluding a fracture tough or malleable layer can be provided byalternating layers of nickel and silica. The silica layers provideprotection from corrosion and the nickel layers provide wear resistanceand fracture toughness. A coating system comprising a nickel layer and asilica layer provide increased corrosion and wear resistance astheorized above.

By layering two materials, namely silica and chromia, the corrosionresistance of the silica can be exploited while the higher thermalexpansion coefficient of the chromia layer acts as a buffer between thealuminum with its high coefficient of thermal expansion (23×10⁻⁶° K⁻¹)and the silica with its low coefficient of thermal expansion (0.55×10⁻⁶°K⁻¹). The silica is deposited to take advantage of its high corrosionresistance. Silica exhibits good corrosion resistance when present as athick coating. However, the present inventor has discovered thatmultiple, thinner layers are better able to withstand more thermalexpansion mismatch than would a single layer of silica of comparablethickness. Silica is a preferred base layer for initial oxidationprotection and for deposition, but other protective materials can alsobe used.

The chromia buffering layers with their intermediate coefficient ofthermal expansion separate the silica layers and help to minimize theexpansion mismatch between silica coating and substrate. Further,nanolaminates having different properties than the bulk materials mayact to passivate the corrosive compounds. The silica layers in theexamples below are composed essentially of oxides of silicon, namelysilicon dioxide, and make up a substantial proportion by weight of themultilayer coatings of the examples since the chromia layers arerelatively very thin.

While silica and chromia were found to be successful with aluminumsubstrates, this particular nanolaminate combination was not aseffective for protecting brass substrates. A literature search formaterials that are used to protect brass yielded a number of potentialcandidates. Several materials were tested. As individual layers, bothlithia doped silica and zinc phosphate were found to provide someprotection. But it was not until these materials were combined as ananolaminate that a significant increase in the amount of protection wasachieved. The coatings were tested using copper-accelerated aceticacid-salt spray/fog testing (hereinafter CASS) using copper chloride.The importance of the multilayer coatings is demonstrated by the factthat if a single layer coating exceeded a few hundred nanometers inthickness the single layer coating would rapidly peel as an individuallayer and lose protection for the substrate.

The individual layers of the coatings of the present invention can bedeposited by any method capable of depositing thin layers onto thesubstrate such as sol-gel, physical vapor deposition and other chemicalvapor deposition techniques. A preferred process of depositing thecoating systems on substrates without detrimentally affecting thesubstrates is a combustion chemical vapor deposition method known asCCVD, described in U.S. Pat. No. 5,652,021 and incorporated herein inits entirety by reference. The use of the CCVD method is ideally suitedto the continuous application of multilayered coatings since only thesolution feed to the deposition apparatus needs to be altered to changethe deposited material. Thus a multilayer coating of alternating ordiffering composition may be quickly and easily deposited. Additionally,the coatings can be applied by the CCVD method to large andcomplicated-shaped articles, such as airplane turbines, propellers, etc.

The CCVD method is a method of applying coatings to substrates usingchemical vapor deposition. The CCVD method is accomplished by mixingtogether a reagent and a carrier solution to form a reagent mixture. Thereagent mixture is ignited to create a flame or the reagent mixture isflowed through a plasma torch in which the reagent mixture is at leastpartially vaporized into a vapor phase. The reagent mixture involves thedirect combustion of flammable liquids or vapors which contain theelements, or reagents to be deposited. The substrate to be coated islocated proximate the flame's end and the vapor phase of the reagent iscontacted to the substrate resulting in deposition of a coating or layerof the reagent. The deposition can be controlled so as to have apreferred orientation of the coating onto the substrate.

Flammable organic solvents containing elemental constituents of thedesired coating in solution as dissolved reagents should be selected forthe CCVD process. The solution is sprayed though a nozzle or vaporizerand burned. The solvent is used to convey the coating reagents and toprovide the combustible material necessary to produce a flame.Alternatively, the vapor reagents can be fed into the flame and burned.Upon burning, the reagent species present in the flame chemically reactand vaporize and then may be deposited onto a substrate held in thecombustion gases or just beyond the flame's end to form a coating on thesubstrate. Longer coating times can be used to produce thicker coatings.

The CCVD process allows the use of a lower plasma temperature thanconventional plasma spraying processes thus not affecting the substratesurface with elevated temperatures. Only enough heat to chemically reactthe reagents is necessary. The reactions occur at lower temperaturesthan necessary to melt the resulting materials as required withconvention plasma spraying processes. Such lower temperatures allows theuse of less expensive, safer, and more mobile equipment yet produce afilm of equal or better quality than most coating methods. Thus, thecoating of surfaces of interior parts and complex shaped parts in thefield may be possible.

The coatings of the following examples, both the single layerComparative Examples and the multilayer examples within the presentinvention were produced in a continuous manner using the CCVD process.In the multilayer examples, Examples 1-4, each of the layers of themultilayer coating system was deposited without interrupting the flame.This was achieved by only altering the precursor feed composition asallowed by the CCVD process. The corrosion resistances of a single layerof chromia on an aluminum substrate, of a single layer of silica on bothan aluminum and a steel substrate and of silica/chromia multilayercoatings on both aluminum and steel substrates were studied and arelabeled as Comparative Examples A-C respectively. The process ofpreparing the coatings and the results of the studies of the coatingsare described in the following Examples.

The multiple layers of the coatings were deposited via the CCVD processusing the following process. Initially, a clean substrate was prepared.Oils and debris should be removed from the substrate before applicationof a coating to allow good adhesion to the substrate and performance.The deposition conditions may need to be determined throughexperimentation depending on the substrate material, geometry andcooling conditions. Low deposition temperatures are often required tominimize or eliminate adversely affecting the substrate properties.However, there are instances where low deposition temperatures are notnecessary and there may be instances where elevated temperatures wouldbe desirable.

The low deposition temperatures can be obtained with short substratedwell times or with cooling of the substrate side opposite the flame,i.e. with water or with air. The first desired precursor solution waspumped to the torch flame, and the first layer of coating material wasdeposited onto the substrate. After the desired deposition time andthickness the sample was moved away from the flame and the solution flowwas switched from one solution to another. Once the new solution wasflowing through the torch, the oxygen flow rate and current settingthrough the torch needle were adjusted to the appropriate values for thesecond solution. The change from one solution to the second solutionflowing through the torch is indicated by a color change in the flame.Once the flame and flame color were stable with the second solutionflow, the sample was moved back in front of the flame at the appropriateposition. This procedure was repeated for the desired number of layers.Alternatively, multiple flames of for example, silica and chromia, couldbe used to deposit the specific desired materials and moved relative tothe substrate to produce a multilayer coating in an assembly line-typefashion.

The metal organic precursor chemicals used consists of the same organiccompounds incorporated in the development of the aforementionedcoatings: tetraethoxysilane (hereinafter TEOS), [Si(OC₂H₅)₄], andchromium(III)acetylacetonate, [Cr(CH₃COCHCOCH₃)₃]. These precursorchemicals were mixed into separate solutions with toluene (methylbenzene), 1-butanol (n-butyl alcohol) and propane. The TEOS andchromium(III)acetylacetonate are to date the best precursor chemicals asfar as deposition and solubility in common solvents. The TEOS solutionconcentration was 0.037M and apart from the precursor chemical, thesolution contained, by volume: 9.12% toluene, 0.87% 1-butanol and 90.01%propane. The chromium acetylacetonate solution concentration was 0.002Mand apart from precursor chemical contained, by volume, 10.58% toluene,4.38% 1-butanol and 85.04% propane. These concentrations were found tobe good but are not absolute for deposition. The concentrations can bealtered to a certain extent to affect the deposition rates. However, therelative amounts of equivalent solvents are recommended to ensuresolution stability. These solutions were prepared to be soluble in eachother by using the same solvents since they come into contact when thesolution flow is switched from one material to the other duringprocessing. If the solutions are not soluble, precipitation of thechemical constituents or other unfavorable reactions could occur whenthey contact each other. Compatibility of silica and chromia depositionsolutions is not needed if separate flames are used.

The environmental concerns associated with the CCVD process used inproducing a nanolaminate are addressed by the combustion process.Oxygen-rich flames are used for complete combustion of the precursor andfor reduced NO_(x) production. Finer atomization improves vaporizationand thus film quality. Extremely fine atomization was obtained by usingthe near supercritical atomization as described in U.S. patentapplication Ser. No. 08/691,853, filed Aug. 2, 1997, entitled “ChemicalVapor Deposition and Powder Formation Using Thermal Spray with NearSupercritical and Supercritical Fluid Solutions” which is incorporatedby reference herein in its entirety. Volatile organic compounds arecombusted to water and carbon dioxide, which may be safely exhaustedthrough a laboratory fume hood. The tetraethoxysilane and chromiumacetylacetonate and any other precursors form the associated oxides ofthe coating, water and carbon dioxide in the flame. The silicates notdeposited are essentially the same material as that which is a componentin common dust. Thus, the preferred method is environmentally safe,producing no hazardous byproducts and can be used outdoors or in avented area.

The improved corrosion resistances of the coatings of the presentinvention are not limited to corrosion resistance in salt-containingenvironments, although test results indicate that the coatings of thepresent invention have excellent corrosion resistance in salt-containingenvironments. The multilayer coatings of the present invention have alsobeen proven effective at elevated temperatures and are expected to beeffective in any other conditions and environments to which silica,chromia or other coating layers in accordance with the present inventionexhibit resistance. The silica/chromia multilayer coatings of Examples1-3 in accordance with the present invention have been shown to providecorrosion protection to a temperature sensitive alloy at a temperatureof 550° C. in air. The multilayer coatings in accordance with thepresent invention provide the corrosion resistance and any beneficialsubstrate oxidation will be incidental.

The present invention provides a coating system that can be deposited ina continuous manner for the corrosion protection of metals, particularlyagainst corrosion in aqueous environments and at elevated temperatures.The effectiveness of the coating system of the present has beendemonstrated by salt water spray testing, salt water/acetic acidimmersion testing and CASS testing and by elevated temperature furnacetesting. Alternatively, the coating system of the present invention canbe applied to articles, surfaces or substrates in order to provide otherbeneficial surface properties, such as surface appearance luster, wearresistance, hydrophobicity, etc.

Using the CCVD process, nanolaminate layers can be deposited which offercorrosion protection to a variety of substrates used in a wide range ofapplications. An additional attribute of the nanolaminate is that byvarying the deposited thickness of the coating or the combination ofelements or compounds used in the multi-layering, the appearance of thefinal product can be controlled. Thus, such processing could providecorrosion-resistant surfaces that have cosmetic or aesthetic finishes.The deposition thickness can be controlled to result in coating with apreferred thin film interference color, or the thickness can be variedover the substrate surface to produce an array of the thin filminterference colors. Layers of metal, e.g. gold, silver and platinum,can also be included in the multi-layering scheme to provide a desiredmetallic luster to the final corrosion-resistant multi-layered coating.

COMPARATIVE EXAMPLE A

Single layer coatings consisting essentially of silica were deposited bythe CCVD process onto aluminum plates and were tested to determine theability of an individual silica coating to protect an underlyingsubstrate in aqueous corrosive conditions. The silica coatings wereobserved to have fairly good corrosion resistance in 5 weight percentNaCl aqueous solutions. However, due to the low coefficient of thermalexpansion of silica coatings relative to the higher coefficient ofthermal expansion of the aluminum substrates to which the silicacoatings were adhered, the silica coatings were subject to crackformation during their development as a coating and would presumably besubject to such crack formation during use at elevated temperatures.These cracks will allow potentially corrosive agents to contact theunderlying aluminum substrates and lead to corrosion of the aluminumsubstrate. Since temperatures of about 100° C. and higher are necessaryin the deposition process, thermal expansion mismatch between thecoating and substrate is a viable concern. Only thin silica coatings canresist the cracking resulting from the thermal expansion mismatch.However, such small thicknesses of a single material are not asdesirable for corrosion resistance.

COMPARATIVE EXAMPLE B

Single layer coatings consisting essentially of chromia were depositedby the CCVD process onto aluminum plates by a similar process to thealuminum plates of Comparative Example A. The chromia-coated aluminumplates were tested to determine the ability of an individual chromiacoating to protect an underlying substrate in aqueous corrosiveconditions. The chromia-coated aluminum plates were tested in a 5 weight% NaCl solution for only up to three days. The specimens' visualappearance were observed to have changed in color. This indicated thatcorrosion of the chromia-coated aluminum specimens had occurred.

COMPARATIVE EXAMPLE C

Additionally, single layer coatings consisting essentially of silicawere deposited by a CCVD process onto iron/cobalt alloy plates using asimilar process and were tested for corrosion resistance at elevatedtemperatures in an oxidizing environment. Iron/cobalt alloy plates arevery susceptible to oxide formation at elevated temperatures. Thesespecimens were tested at elevated temperatures of at least 400° C. Thesilica only coatings were not successful in preventing oxidation of theunderlying iron/cobalt substrates.

Since the results of the testing of the single oxide layers wereunsatisfactory, alternating layers of silica and chromia were proposed.The following Examples showed very good corrosion resistance in aqueouscorrosive environments, at least 30 days in an aqueous 5% NaCl solutionat a pH of 3. The multilayer coatings in accordance with the inventionshowed superior corrosion protection compared to single layers. In fact,a more acidic test solution was used to test the multilayer coatingscompared to the solution used to test the above single layer coatings.Additionally, the multilayer coatings in accordance with the inventionshowed superior corrosion protection at elevated temperatures. Thetemperature susceptible iron/cobalt alloy was adequately protectedduring exposure to air at 550° C. for 12 hours when coated with asilica/chromia multilayer whereas the silica only coating was not ableto provide protection at 400° C. The details of the coatings areprovided below.

EXAMPLE 1

Multilayer coatings of silica and chromia were deposited onto aluminumusing the CCVD process. During the deposition, the solution flow rate,oxygen flow rate and cooling air flow rate were kept constant. The flowrate for the solution was 3.0 ml/min. and for the oxygen 3000 ml/min. at65 psi. The cooling air was at ambient temperature and the flow rate was9800 ml/min. at 80 psi. The cooling air flow rate was directed at theback of the substrate with a copper tube whose end was positioned oneinch from the back of the substrate. Two deposition parameters werechanged with each changeover in solution. These included the flametemperature at the substrate surface as measured with a type Kthermocouple positioned in line with the flame close to the specimensurface (700° C. for the silicon solution and 650° C. for the chromiumsolution) and the current passing through the torch needle as controlledwith a variac (about 3.0 amps for the silicon solution and 2.5 amps forthe chromium solution). The flame from the silicon solution was bluewhile the flame from the chromium solution was pink.

The solution flowing to the torch was changed five times for a total ofthree layers of silica (layers 1, 3 and 5) and two layers of chromia(layers 2 and 4). Additionally, four and six layer samples wereproduced. The four layer samples comprised a substrate and sequentiallayers of silica, chromia, silica and chromia. The six layer samplescomprised a substrate and sequential layers of silica, chromia, silica,chromia, silica and chromia. The size of the rectangular aluminum platesubstrate, 3 cm×5 cm, required that flame move along a pattern to coatall of the substrate evenly. Thus, the sample was moved in a manner sothat the flame followed the perimeter of the specimen. This produceduniform coatings as observed by the thin film interference colors. Eightlaps were performed for each of the two solutions for each of theseparate layers. The two different solutions were applied at differentspeeds. The silica solution was deposited in just over 14 minutes forthe eight laps while the chromia solution was deposited in just over 13minutes for the eight laps. During the deposition, the thin filminterference colors eventually were no longer distinguishable as thelayers were deposited. The thicknesses of the individual silica layersvaried form about 0.3 microns to about 0.5 microns and the thickness ofthe individual chromia layers was less than 0.05 microns. The totalthicknesses of the four layer samples varied from about 0.6 microns toabout 1 micron and the five and six layer samples from about 1 to about1.5 microns. The final color of the specimens with the five film layerswas gold/yellow.

The samples were initially tested for corrosion resistance in an aqueoussolution of 5% NaCl with a pH of approximately 3, attained with anappropriate addition of acetic acid. The samples were tested byimmersion in the solution such that the liquid reached half way up thesample's long dimension. Only one of the twelve tested samples inaccordance with this Example was observed to have any noticeablecorrosion.

Energy dispersive x-ray (hereinafter EDX) analysis revealed that thecoating was high in silicon, possibly indicating that the three layersthat are visible in the micrograph are all silica and that the chromialayers are either too thin to be observed, dissolved or just not visiblein this picture or at this magnification. The chromia layers wereintentionally deposited as very thin coatings compared to the silicalayers, so their appearance in the SEM as separate layers would not beexpected. The chromium solution concentration was almost one third ofthat used to form the individual chromium oxide coatings described inthe Comparative Examples above. EDX analysis did indicate the presenceof chromium, so the material was deposited. Numerous aluminum substrateswith the same multilayer coating were also tested by Behr Automotive ofStuttgart, Germany, also using a salt water/acetic acid immersion test.After 30 days of immersion testing, the coatings were observed to haveonly a few microscopic pores. These microscopic pores are consideredwithin acceptable limits.

EXAMPLE 2

The same silica/chromia multilayer coating described above that wasdeposited onto aluminum plate was also deposited onto 4 inch by 6 inch,aluminum-coated carbon steel plates. The aluminum-coated surface ontowhich the deposition occurred had more surface roughness than thealuminum plates of Example 1. These specimens were tested by anindependent testing laboratory, Battelle of Columbus, Ohio, in a saltspray chamber. These multilayer coated, aluminum-coated plates withstood1344 hours of testing without any noticeable signs of corrosion, thus,surpassing Military C-83488 Specifications (672 hours minimum). Thissubstrate with the multilayer was also tested in the 3 pH salt immersiontest and lasted over two weeks without any visual evidence of corrosion.Conversely, a bare substrate without coating was observed to have visualsigns of corrosion in one day.

EXAMPLE 3

A similar silica/chromia multilayer coating as described above wasdeposited onto an iron/cobalt alloy in order to determine elevatedtemperature corrosion resistance. The composition of the layers wasidentical as above Examples 1 and 2 but only three layers were used,silica/chromia/silica. Coated and uncoated specimens were tested in atube furnace at a minimum of 400° C. for 12 hours in the air. The coatedspecimens were protected from corrosion at 550° C. with some change ininterference colors and no or little evidence of substrate oxideformulation in localized areas, even when the upper layer experiencedpeeling and spallation due to too great a thickness. Uncoated substratesshowed extensive corrosion as exhibited by oxide formation anddiscoloration.

EXAMPLE 4

Brass is a metal susceptible to corrosion in salt water solutions by theleaching of zinc from the alloy, i.e. dezincification. Thesilica/chromia multilayer described in Example 1 was deposited ontobrass specimens measuring approximately 1.5 inch by 2 inch and 0.1 inchthick. The silica/chromia multilayer-coated samples were tested in anaccelerated salt test solution as described in the standard test ASTMB368-85, not in a fog but by immersion. This solution consisted of 5parts by weight sodium chloride, 95 parts by weight distilled water,0.25 g reagent grade copper chloride per liter of salt solution andacetic acid added to reach a solution pH of 3.1 to 3.3. Thesilica/chromia multilayer-coated samples corroded in the corrosionsolution. Therefore, a new coating was developed to deposit onto brassfor corrosion protection in the described solution.

Single and multilayered coatings of lithia doped silicon oxide (1 atomic% Li) and undoped silicon oxide and zinc phosphate (ZnPO₄) weredeposited onto brass specimens as described above. The silicon precursorwas tetraethoxysilane in toluene (1.5 weight % Si), the lithiumprecursor was Li t-butoxide in methanol (0.6 weight % Li), the zincphosphate precursors were zinc 2-ethylhexanoate in toluene and mineralspirits (1 weight % Zn) and triethylphosphate in toluene (1.7 weight %P), respectively. The Li-doped silica solution had a final solutionweight percent of 0.0007 silicon and 0.000002 lithium, with 11.2 addedweight percent of toluene and 83.8 added weight percent of instrumentgrade propane. The zinc phosphate solution had a final solution weightpercent of 0.0002 zinc and 0.0002 phosphorous, with 3.3 added weightpercent of toluene and 92.8 added weight percent of instrument gradepropane.

During the deposition, the solution flow rate and oxygen flow rate werekept constant. The flow rate for the solution was 3.0 ml/min and for theoxygen about 4.0 L/min at 80 psi. The cooling air was at ambienttemperature and the flow rate was varied between 2.5 and 12.8 L/min at90 psi. For the five layered samples, the flow rate was 12.8 L/minduring the first layer (Li-doped silica) and either 7.5 or 2.5 L/min forthe four subsequent layers. The current passing through the needle wasabout 2.5 Amp for silica, about 2.9 Amp for Li-doped silica and about2.7 Amp for the zinc phosphate. The gas temperature at the surface ofthe sample for all deposition materials was 700° C.

The number of layers deposited was one, two and five. The silica (dopedor undoped) was deposited as a single layer, as a base layer withanother layer applied on top and, when doped, as layer(s) 1, 3 and 5 ofa multilayered coating. The zinc phosphate was deposited as a singlelayer, as a second layer with a silica base layer and as layers 2 and 4in a multilayered coating. The thickness of each distinct layer was lessthan about 100 nanometers The sample size dictated that the sample moveduring the deposition. The program moved in the same pattern and at thesame rate for each material; only the time (or number of motion programlaps) of the deposition varied, thus, resulting in different thicknessesof deposited material. In general, the zinc phosphate deposition timeper layer was approximately three times that of the silica depositiontime.

All the samples were tested in the ASTM B 368-85 solution except for onesample which consisted only of zinc phosphate on an aluminum plate. Thissample was tested in a solution as above but without the copperchloride. Only minor corrosion was observed after 17 days. All samples(aluminum and brass) were immersed in the test solution. Only one sideof each sample was coated and the uncoated side of each sample wascompletely covered with tape to prevent contact with the test solution.

Brass samples coated with just Li-doped silica showed better corrosionresistance in the corrosion solution than when coated with undopedsilica. Brass samples coated with just zinc phosphate and a silica baselayer also showed good corrosion resistance. However, when these twocoatings were combined into multilayers on a brass sample, the corrosionresistance improved. One sample coated with an undoped silica base layerand a top layer of zinc phosphate was immersed in the corrosion solutionand showed no signs of corrosion after two months; the edges exhibitedcorrosion but they were not purposefully coated. Ten samples coated withLi-doped silica as layers 1, 3 and 5 and with zinc phosphate as layers 2and 4 were immersed in the above corrosion solution for two weeks andshowed no signs of corrosion except at the uncoated edges of some of thesamples.

EXAMPLE 5

Multilayer coatings of silica and ceria were deposited onto AISI 1010steel substrates using a CCVD process. During the deposition of thelayers of the coating, the solution flow rate and oxygen flow rate werekept constant. The flow rate for the solution was 3.0 ml/min. and theflow rate for the oxygen was 4000 ml/min. at 80 psi. The cooling air wasat ambient temperature. The flow rate for the silica was varied from18.9 to 21.9 liters/min. and the flow rate for the ceria was varied from12.8 to 18.9 liters/min. The cooling air flow rate was directed at theback of the substrate with a cooper tube whose end was positionedapproximately one and a half inches from the back of the substrate. Theflame temperature at the substrate surface was 700° C. for the silicaand 750° C. for the ceria. The current passing through the needle was2.5 amps for silica and 2.6 amps for ceria.

The silica solution had the same concentration as that mentionedpreviously for the TEOS precursor solution but did not contain1-butanol. The ceria solution was composed of Ce-2-ethylhexanoate intoluene, comprising approximately 1.8 weight percent cerium withadditional toluene and propane. The overall ceria solution concentrationwas 0.001M.

The solution flowing to the torch was changed four times for a total oftwo layers of silica (layers 1 and 3), each silica layer about 70 nmthick, and two layers of ceria (layers 2 and 4), each ceria layer about60 nm thick. Five samples were coated onto 1.5 inch by 1.5 inch steelplates. Each sample was moved in a manner so that the flame followed itsperimeter. The coating time for silica was approximately 5 minutes perlayer and for the ceria was approximately 20 minutes per layer for fourof the samples. The fifth sample had a ceria coating time of 10 minutesper layer. The total coating thickness was less than 1 μm in all cases.

The samples were tested in a salt spray chamber with a 5% NaCl saltsolution with a pH of approximately 6.5 as measured with 0.3 intervalindicator paper. The salt spray chamber temperature was 95 to 96° F. Allthe samples survived at least 170 hours in the salt fog without anysubstrate rust on the coated area (except for one small, pin-hole sized,isolated spot on one sample). The film interference colors were stillapparent with only some film color change.

The above Detailed Description of the Preferred Embodiments and Examplesare presented for illustrative purposes only and are not intended tolimit the spirit and scope of the present invention, and itsequivalents, as defined in the appended claims. It is to be understoodthat the foregoing relates to particular embodiments of this inventionand that numerous changes may be made without departing from the scopeand spirit of the invention.

What is claimed is:
 1. A multilayer inorganic coating on a metalsubstrate comprising: a first distinct layer of a first inorganiccomposition disposed over the substrate, wherein the first distinctlayer has a thickness that is not greater than about 10 microns, and asecond distinct layer of a second inorganic composition disposed overthe first distinct layer, wherein the second distinct layer has athickness that is not greater than about 10 microns and either the firstdistinct layer or the second distinct layer is corrosion-resistant, andwherein the second distinct layer comprises zinc phosphate, an oxide ofchromium or an oxide of cerium.
 2. The coating of claim 1, wherein thefirst layer comprises silicon dioxide and the second layer compriseschromium(III)oxide.
 3. The coating of claim 1, wherein the first layercomprises silicon dioxide and the second layer comprises zinc phosphate.4. The coating of claim 1, wherein the first layer comprises lithiadoped silicon dioxide and the second layer comprises zinc phosphate. 5.The coating of claim 1, wherein the first layer comprises silicondioxide and the second layer comprises ceria.
 6. The coating of claim 1,wherein the first distinct layer has a thickness of not greater thanabout 200 nanometers and the second distinct layer has a thickness ofnot greater than about 200 nanometers.
 7. A multilayer inorganic coatingon a metal substrate comprising: a first distinct layer of a firstinorganic composition disposed over the substrate, wherein the firstdistinct layer has a thickness that is not greater than about 10microns; a second distinct layer of a second inorganic composition overthe first distinct layer, wherein the second distinct layer has athickness that is not greater than about 10 microns and either the firstdistinct layer or the second distinct layer is corrosion-resistant; athird distinct layer of a composition different from the composition ofthe second distinct layer disposed over the second distinct layer,wherein the thickness of the third distinct layer is not greater thanabout 10 microns; a fourth distinct layer of a composition differentform the composition of the third distinct layer disposed over the thirddistinct layer, wherein the fourth distinct layer has a thickness notgreater than 10 microns; and a fifth distinct layer of a compositiondifferent from the composition of the fourth distinct layer disposedover the fourth distinct layer, wherein the fifth distinct layer has athickness not greater than 10 microns, and wherein the first distinctlayer, the third distinct layer and the fifth distinct layer comprise anoxide of silicon.
 8. The coating of claim 7, wherein the second distinctlayer and the fourth distinct layer comprises zinc phosphate, an oxideof chromium or an oxide of cerium.
 9. The coating of claim 1 wherein thefirst layer comprises silicon dioxide.
 10. The coating of claim 1wherein said first layer comprises lithia doped silicon dioxide.
 11. Thecoating of claim 1 wherein the thickness of said first layer is notgreater than about 0.5 microns.
 12. The coating of claim 11 wherein thethickness of said second layer is not greater than about 0.5 microns.13. The coating of claim 1 wherein the thickness of said second layer isnot greater than about 0.5 microns.
 14. The coating of claim 1 whereinthe thickness of said first layer is not greater than about 200nanometers.
 15. The coating of claim 1 wherein the thickness of saidsecond layer is not greater than about 200 nanometers.
 16. The coatingof claim 1 wherein said first distinct layer is deposited over saidsubstrate by combustion chemical vapor deposition and said seconddistinct layer is deposited over said first distinct layer by combustionchemical vapor deposition.
 17. The coating of claim 1 further comprisinga third distinct layer of a composition different from the compositionof said second distinct layer disposed over said second distinct layer,wherein said third distinct layer has a thickness not greater than about10 microns.
 18. The coating of claim 17 further comprising a fourthdistinct layer of composition different from the composition of saidthird distinct layer disposed over said third distinct layer, whereinsaid fourth distinct layer has a thickness not greater than about 10microns.
 19. The coating of claim 18 further comprising a fifth distinctlayer of composition different from the composition of said fourthdistinct layer disposed over said fourth distinct layer, wherein saidfifth distinct layer has a thickness not greater than about 10 microns.20. The coating of claim 19 wherein said first distinct layer, saidthird distinct layer, and said fifth distinct layer each comprisesilicon dioxide.
 21. The coating of claim 20 wherein said seconddistinct layer and said fourth distinct layer each comprise a materialselected from the group consisting of zinc phosphate, chromium oxide,and cerium oxide.
 22. The coating of claim 21 wherein each of saidfirst, second, third, fourth, and fifth distinct layers has a thicknessless than about 400 nanometers.
 23. The coating of claim 1 wherein thematerial of said first distinct layer has a coefficient of thermalexpansion that is lower than the coefficient of thermal expansion ofsaid substrate, and the material of said second distinct layer has acoefficient of thermal expansion that is higher than the coefficient ofthermal expansion of the material of said first distinct layer.